Vibrio cholerae (V. cholerae) of O group 1 cause the disease cholera, a well-recognized cause of morbidity and mortality throughout the world. To date, there have been seven recorded pandemics of this severe dehydrating diarrheal disease (Barua et al, Eds., Cholera, pgs. 1-36, Plenum Medical Book Company, New York (1992)). Diarrhea caused by V. choleraeO1 is due to the action of cholera toxin. Mortality without medical treatment can be as high as 70% (Barua et al, supra).
V. cholerae of O groups other than 1 (non-O1, or non-agglutinating (NAG) V. cholerae) also cause gastrointestinal disease (Spitell et al, Eds., Clinical Medicine, Harper and Row, Philadelphia (1991)); and Morris et al, Epidemiol. Rev., 12:179-191 (1990)), as well as extraintestinal infections, such as wound infections and septicemia (Morris et al, supra; and Safrin et al, Rev. Infect. Dis., 10:1012-1017 (1987)).
Until recently, it was believed that only O1 strains have epidemic potential, with non-O1 strains being limited to sporadic cases and small outbreaks. However, in late 1992 a large outbreak of severe cholera-like disease started in Eastern India (Albert et al, Lancet, 341:704 (1993); Ramamurthy et al, Lancet, 341:703-704 (1993); Jesudason et al, Lancet, 341:1090 (1993); and Albert et al, Lancet, 342:387-390 (1993)), and spread to Bangladesh in January 1993 (Chongsa-nguan et al, Lancet, 342:430-431 (1993)). By April, 1993, more than 15,000 people had been affected, and at least 700 had died from this epidemic (Ramamurthy et al, supra; and Chongsa-nguan et al, supra). Only non-O1 V. cholerae were isolated from a majority of the patients. These isolates have since been typed and designated O139 synom. Bengal.
The current epidemic is spreading rapidly, and V. cholerae O139 Bengal is replacing V. cholerae O1 in affected areas (Chongsa-nguan et al, supra). Tracking the spread of the epidemic requires a means of determining whether persons have been exposed to this new strain of V. cholerae. Further, correct diagnosis of infected persons, and effective public health interventions will depend on an effective means of rapidly identifying the causative agent in environmental or clinical samples.
Previous exposure to V. cholerae can be determined by testing serum samples for specific antibodies induced by the infection. During O1 V. cholerae infection, antibodies against a variety of antigens are induced, with lipopolysaccharide (LPS) and cholera toxin (CT) eliciting the highest titers (Levine et al, Microbiol. Rev., 47:510-560 (1983); and Kaper, Rev. Infect. Dis, 11:S568-S573 (1989)). While V. cholerae O1 and V. cholerae O139 Bengal have been found to be very closely related phylogenetically, and share many antigens, including CT, there are antigenic differences in the LPS (Albert at al, supra; Ramamurthy et al, supra; and Jesudason et al, supra). It has been found that V. cholerae O139 Bengal LPS is truncated. Therefore, there may be problems with immunogenicity of the O139 Bengal LPS, as well as cross-reactivity with O1 LPS. Accordingly, there is a need to find and provide additional antigens which are specific for V. cholerae O139 Bengal infection, and which can be identified quickly and easily.
In contrast to V. cholerae O1, it was found in the present invention that V. cholerae O139 Bengal produces a polysaccharide capsule. This was an unexpected finding for a strain causing epidemic cholera, as V. cholerae O1 are not encapsulated. That is, capsules have previously been seen only on non-epidemic strains of V. cholerae. Further, in contrast to V. cholerae O139 Bengal, these non-epidemic strains generally do not produce CT, and, in the past, have only been associated with sporadic illness. Thus, the discovery and purification of the capsular polysaccharide of V. cholerae O139 Bengal in the present invention provides, for the first time, the ability to quickly and easily identify O139 Bengal infection.
Moreover, the age distribution of the current O139 Bengal epidemic indicates that previous infection with O1 strains of cholera does not induce protective immunity against the new strain. Therefore, cholera vaccines currently being developed against V. cholerae O1 will not be effective against the new threat caused by V. cholerae O139 Bengal (Chongsa-nguan et al, supra; and Albert et al, supra). Thus, the discovery and purification of the capsular polysaccharide of V. cholerae O139 Bengal in the present invention provides, for the first time, the ability to produce vaccines effective against O139 Bengal infection.